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[wilf]

Tim nz, all engine mounts are tight, timing checked and found to be a tad advanced. Adjusted accordingly. Carb is standard mîcarb with 115 màin jet, presumably to compensate for sporty silencer fitted by PO. I'll give her a run tomorrow and see if there's any improvement. I can see myself putting the speedo fast again, just so I think I'm doing sixty!

[Presto]

I’d hazard a point of view, that if an engine is ‘running out of breath’ at full throttle there will be a noticeable pick-up in liveliness when the throttle is eased back a little. It’ll also feel ‘exhausted’; in other words, there will be a discernible drop in power and a flattening out of revs at full throttle, instead of a steady continuing acceleration and willingness to rev-out. The engine won’t ‘pull’ any harder. It’s another point, and more difficult to decide, if this ‘fall-off’ is due to a main jet that’s too small or an air supply that’s restricted. Either cause can produce generally similar effects. In general terms, and up to a point, more air/more fuel = greater thermodynamic efficiency and better performance. (What does worry me here is that wilf’s 500 is only giving an easy 45mph. I’d have thought that was some way below normal for an untuned 500.)

[Tim NZ]

IF, by slightly rolling back the throttle from full, there is a momentary improvement in performance, the main jet is too small.


ANY hesitation or instantaneous misfire when the throttle is snapped fully open from 3/4 throttle, the main jet is way too small.


Plug chop reveals all. Better to start out Rich than Lean, as a fouled plug is easier and cheaper to remedy than a holed piston or a burnt Ex valve.

[Presto]

I’m going to disagree with Tim NZ on a couple of points. A plug chop doesn’t reveal all, it hardly reveals anything significant in a road-going situation (racing is different; there a plug chop from a sustained flat-out run can tell you something). But with modern fuels, a plug-chop in a road-going situation, tells you next to nothing. The other point was that snapping to full-open throttle is likely to produce a hesitation or misfire or engine ‘cough’ even when the mixture is correct. The reason is the greater inertia of fuel over air. The snapped-open throttle allows a rush of air, fuel can’t react with equal speed, so there’s a momentary weak mixture. Some carbs compensate for this (DellOrto, for example) with an accelerator pump, injecting neat fuel when the throttle is opened suddenly. In the case of carbs without that devise the answer is simple – don’t snap the throttle open, open it progressively.

[Revband]

Sorry Presto

Read Tim NZ's post again, he is absolutely correct!, on the plug chop you do of course need to know what you are looking for, and he states opening the throttle ftom 3/4 to full if you bike won't do this then you have a problem.

[Presto]

Hi Revband - Tim did specifically write when the throttle is 'snapped' open from 3/4 to full. In such a case hesitation may be 'normal' with a carb with no accelerator jet (that's why other carbs go to the trouble of fitting them). I don't agree that a plug chop (in road-going situations) will show you much useful information now fuel includes ethanol. It's not a case of knowing what you're looking for, it's a case of not being able to see what won't be there to see. But each to their own view - I'll let it drop.

[Tim NZ]

Amal and Mikcarbs are both Air bleeding carbs: the needle jet sits immersed in a ‘well’ of petrol; when the throttle is snapped open, the petrol that is in the annular space surrounding the jet is drawn into use and has much the same enrichening effect as a ‘fuel pump’.


If the bleed holes in the needle jet become blocked with varnish, rust, scale etc, snap acceleration suffers.


The Spark plug is the Window into the engine and reveals ALL, no matter what the throttle position for both the carburetion and ignition timing! Unsure how to read a spark plug? Start by going too rich, the plug will eventually carbon up. Raise the float level to 26mm? Go up 4 main jet sizes? Leave the choke on…


To see fell the effects of momentary leanless, simply ride the bike at any given fixed throttle position and turn off the petrol. (It wont harm your motor)


Looking for ‘colour’ on your spark plug? Don’t waste your time. Unleaded and ethanol blends DO NOT colour spark plugs. (Never did, never will!) ANY colouration (other than Black) is a result of fuel additives and or Oil contamination in the combustion chamber and has NO direct relevance to the mixture. Not that the mythical orange/tan colour was ever anything other than a glaze that indicated optimal plug operating temp and had next to nothing to do with correct mixture.




What is one looking for?


A fine layer of SOOT extending up from the base of the central electrode to about half way up the insulation.


What do you NOT want to see? A perfectly clean central insulation with ANY sign of speckling or blisters! They are bits of your melting/ablating piston!


Plug nice and clean with part throttle, light loads and moderate rpm? Carburetion and the plugs heat range are quite forgiving in such situations and a clean plug should be expected. A touch of soot on the plug shell is all one should expect. Want to see what happens when a plug is too ‘cold’? Fit one and find out! Same with too hot, but be careful, as too hot a plug WILL eventually lead to pre-ignition!


Whilst for racing purposes it is good to do a full throttle peak revs plug check, it is not so critical for a road bike, simply pick your biggest steepest longest hill and Load-up the motor. ie Full throttle in 3 rd gear motor pulling hard, +4000rpm, 60mph…


No big hill around? Do the run into a stiff head wind with a pillion on board…
If in doubt run rich! It is cheaper and easier to clean a fouled spark plug that it is to replace a holed piston.

[Presto]

Ok, I’ll post a brief reply, because there seems to be confusion over what we’re talking about. (And it's fun!) Fuel inertia still applies, even if the carb is an ‘air bleeding’ type. The laws of physics still apply. Fuel has greater inertia than air. Air will flow through a carb more readily than fuel. Snapping open the throttle allows a sudden rush of air that the fuel, momentarily, cannot equal. Result: temporary weak mixture.

To read a spark plug is one thing, to rely on a ‘plug chop’ on a road-going bike is another. To be sure, the state of a plug will tell you something, may tell you a lot, about how an engine is running. But not as the result of a sudden ‘plug chop’ after a few yards at a ‘stable’ speed. The condition of a plug’s firing tip does not change at such a rapid rate. The state of a plug tip may show the general state of engine tune. I don’t believe it will show how an engine is running, at a given throttle setting, for a few yards along the road.

[wilf]

Thanks lads, went for a spin this evening with correctly adjusted timing and felt a real difference. Tinker now accelerares smoothly up to a full throttle speed of sixty without any sign of having a panic attack. As no one has said anything particularly positive about sports air filters I think I'll give that a miss.
Special thanks to Tim NZ and Presto for the knowledgeable debate on the inner workings of carburettors. Admirably clever stuff.
Cheers, Wilf.

[Tim NZ]

For future reference, keep in the back of your mind that for each 1mm you move the points plate, you are advancing/retarding the ignition timing by nearly 3 degrees...